Value of attribute ... invalid: ... cannot start a name
Cause:
Each attribute belongs to a category. And for some types, their values are restricted. Here are the categories of attributes:
- CDATA is a sequence of characters
- ID and NAME must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").
- IDREF and IDREFS are references to ID tokens defined by other attributes. IDREF is a single token and IDREFS is a space-separated list of tokens.
- NUMBER tokens must contain at least one digit ([0-9])
Example:
<a> attribute "id" has invalid value "567"
The attribute ID is of type ID. As described above, it should begin with a letter
 | <a id="567" href="/index.html"> |
 | <a id="_hello" href="/index.html"> |
 | <a id="n567" href="/index.html"> |
Solution:
To find the type of the attribute, search the tag name (here) and check its definition.
References: